Narset Transcendent has a casting cost of 2WU (2 colorless, 1 White, 1 Blue) and enters the battlefield with 6 six loyalty counters. That's an unusually high number for a 4 mana planeswalker. Considering her abilities, that loyalty is quite relevant.
Her +1 ability allows you to look at the top card of your library. If it's a noncreature, nonland card, you may reveal it and add it to your hand. Narset's -2 ability gives the next instant or sorcery spell you cast rebound. This means that you get to recast that spell for free during your next upkeep. These first two abilities enable Jeskai clan mechanics like Prowess and the effects of Jeskai Ascendancy.
The final ability that Narset the Planeswalker possesses, often called an "ultimate" ability, gives you an emblem. For the cost of 9 loyalty counters, your opponents can't cast noncreature spells for the rest of the game. This may be one of the most crippling emblems ever created. It's hard to say how often this ability will resolve given the usefulness of the first two abilities, but making it so that your opponents can only ever again play creatures and lands for the remainder of the game is extremely limiting.
Planeswalkers aren't played simply for their ultimate abilities, but Narset Transcendent has changed the game in Commander a great deal. The popular Super Friends Commander deck will definitely find a home for this Narset. Super Friends decks are three-color or five-color decks that revolve almost entirely around the effects of planeswalkers. Not only does she help your get your planeswalkers (which are non-creature spells) and other useful trinkets like Enchantments and artifacts, but she can simply take all other players out of the game. With an Enchantment card like Doubling Season, which doubles counters, Narset reaches 12 loyalty counters as soon as she enters the battlefield. If she and Doubling Season are on the board at the same time, Narset's ultimate ability basically wins the Super Friends player the game. Of course, Narset will be played all over the Commander format simply for her ability to get a free card from the top of your deck and to give instant and sorcery bomb spells rebound.
Narset Transcendent is going to be an extremely popular planeswalker with a lot of playability. Jeskai decks (previously known as Team America for their red, white, blue color combination) everywhere will make room for her. She may become one of the top planeswalkers in the game.